The present invention relates to preparation of semiconductor alloys of the formula Hg.sub.1-x Cd.sub.x Te.sub.1 generically referred to as "HgCdTe". Such alloys are needed for infrared detectors. The materials requirements of such applications are:
(1) compositional uniformity, i.e. constant value of the alloying parameter x; (2) low concentration of impurities (ideally less than 10.sup.14 /cm.sup.3); and (3) good crystal quality (low density of vacancies and of dislocations).
Heretofore, there has been little effort to reduce or control dislocation density of Hg.sub.1-x Cd.sub.x Te. Bulk material grown by the quench anneal or solid state recrystallization process typically exhibits dislocation densities in the range of 10.sup.5 -10.sup.7 cm/cm.sup.3, while epitaxial materials can show similar or slightly lower densities. The as-grown material usually includes an excess of Te (typically very much less than 1% atomic), and generally exhibits a high concentration of metal vacancies, the concentration of which is controlled by annealing under Hg saturated conditions usually below about 300.degree. C. The annealing process as presently practiced throughout the industry can actually increase the dislocation density by about a factor of ten.
A major difficulty in the prior art preparation of HgCdTe can be seen in the phase diagram of FIG. 1, which shows the prior art process. (The pseudo-binary phase diagram is slightly sensitive to the Hg:Cd ratio, but the Schematic diagrams of FIGS. 1 and 2 are fairly accurate for the HgCdTe compositions of primary interest, viz. 4 to 12 micron material.) That is, the as-compounded material is recrystallized at a fairly high temperature T.sub.1. This solid state recrystallization process is well known, and is shown, for example, in vol. 18, pp. 48-119, of Semiconductors and Semimetals. As is well known in the art, the recrystallization process should take place at a temperature T.sub.1 which is between 600.degree. and the melting point of the solid, which is approximately 680.degree. C. The recrystallization step requires a fairly long time, preferably forty hours or more. This recrystallization step homogenizes the material, that is the compositional parameter "x" in the alloy Hg.sub.1-x Cd.sub.x Te becomes uniform throughout the material.
After this recrystallization step, the HgCdTe material, which is now compositionally uniform, is conventionally cooled to room temperature, where the ingot is sliced into bars whose physical dimensions approximate those of the final material used for device fabrication. These bars are then heated to a lower temperature T.sub.2 for a so-called post anneal step in mercury vapor. The temperature T.sub.2 is typically 300.degree. or less, and annealing at this temperature is performed, in the prior art, for a time of at least an hour (and typically much longer) in an ampoule containing some liquid mercury. This post anneal step approximately restores the HgCdTe material to stoichiometry, which is shown in the phase diagram of FIG. 1 by the vertical dotted line at 50 atomic percent on the composition axis.
However, the chief difficulty of this process can also be seen from FIG. 1. That is, when the material is cooled down from the recrystallization temperature T.sub.1, it moves from the single phase region, where the desired HgCdTe crystalline structure is found, into a two-phase region, where precipitates of tellurium are also present in the HgCdTe matrix. Since tellurium is highly immobile in HgCdTe, these tellurium precipitates are a source of trouble. That is, during the post anneal stage, when the composition of the material is changed at a constant temperature T.sub.2, the mercury which must indiffuse to effect the necessary compositional change, will combine with the precipitated tellurium.
However, this causes a substantial multiplication of existing dislocations in the matrix lattice, reducing the resulting crystal quality. Additional background is found in Anderson et al., "Precipitation and Phase Stability of (Hg,Cd)Te", 21 J. Vac. Sci. Technol. 125 (1982), which is hereby incorporated by reference.
The method of the present invention teaches annealing slices of Hg.sub.1-x Cd.sub.x Te under a mercury saturated ambient, at temperatures (e.g. 600.degree. C.) where dislocation motion can occur by climb, for a period sufficient to allow the depletion of dislocations adjacent to the surface and to allow excess tellurium present to be removed by the indiffusion of mercury. For example, at 600.degree. C. about four hours are required to minimize the dislocation density near the surface. Followiing this treatment the slices can be post annealed under saturated mercury conditions below 300.degree. C., without inducing dislocation multiplication which is normally encountered without use of this invention.
According to the present invention there is provided: a method for preparation of approximately stoichiometric alloys of (Hg,Cd)Te having a preselected proportion of mercury to cadmium, comprising the steps of: providing a bulk alloy of mercury, cadmium, and tellurium, said bulk alloy containing an excess of tellurium over said predetermined composition; annealing said bulk alloy, at an intermediate temperature which is greater than 350.degree. C. and less than the melting point of said bulk alloy, in an overpressure of mercury, whereby the composition of said bulk alloy is converted to an intermediate alloy having a smaller excess of tellurium.